Condenser microphones are electroacoustic transducer apparatuses including electroacoustic transducers that convert acoustic waves to electrical signals. An electroacoustic transducer of a condenser microphone includes a diaphragm ring, a diaphragm, a spacer, and a fixed electrode. The diaphragm is stretched on the diaphragm ring with predetermined tension. The fixed electrode is disposed adjacent to the diaphragm with the spacer disposed therebetween.
An electrostatic attraction force is applied to a diaphragm of a condenser microphone operated by a DC bias even when no sound pressure is applied. Such an electrostatic attraction force causes the diaphragm to undesirably stick to the fixed electrode. The contact of the diaphragm with the fixed electrode should be avoided even when high sound pressure is applied to the diaphragm.
The limit of the sound collection in a low frequency band of a first order pressure-gradient microphone depends on the tension of the diaphragm. That is, when the tension of the diaphragm increases, the limit of the sound collection in a low frequency band shifts to a high frequency region. On the other hand, when the tension of the diaphragm decreases, the limit of the sound collection in a low frequency band shifts to a low frequency region. As a result, the frequency response of the diaphragm in the low frequency band is to be improved, and the counteract force of the diaphragm against the adsorption force to the fixed electrode is to be weakened.
Schemes have been proposed to improve the frequency response of the diaphragm in a low frequency band and to improve the counteract force of the diaphragm against the adsorption force to the fixed electrode (for example, refer to Japanese Patent No. 5055203).
In general, a diaphragm is composed of a thermoplastic resin film such as a polyethylene terephthalate film or a polyphenylene sulfide film. The thermoplastic resin film is a biaxially stretched film stretched in the longitudinal and lateral directions. Thus, the mechanical properties, such as tensile strength, of the diaphragm in the longitudinal and lateral direction, and the temperature dependency of the mechanical properties of the diaphragm depend on the stretched direction of the stretched film.
The diaphragm proposed in Japanese Patent No. 5055203 is prepared through corrugating. The diaphragm having a pattern with a large-pitched ridges and grooves achieves the same advantageous effects as those of a diaphragm having multiple ribs. Thus, the diaphragm operates in the same manner as a diaphragm having multiple small compartments. As a result, the mechanical anisotropy of the stretched film of the diaphragm becomes small. Thus, even in rectangular diaphragms, the individual differences in the natural resonance frequencies of the individual diaphragms becomes small through matching of the stretched direction of the stretched film with the extending direction of the long or short side of the diaphragms, in a production process of the diaphragms.